The present invention relates to an improved method of making fluoride-base glass free of certain contaminants, and in particular, free of carbon dioxide and chlorine. Considerable effort has been expended to develop heavy metal fluoride glasses ("HMFG") as a viable family of infrared-transparent optical-materials for application as fiber waveguides and as bulk optical components. The specific characteristics of these materials and their advantages in terms of optical performance over silica and other oxide-based glasses are well known.
Because of fundamental physio-chemical differences, the HMFG are not as easily formed into bulk components (e.g., plates, discs, rods) as are many oxide-based glasses. For example, the viscosity (0.1-1 poise) of most heavy metal fluoride melts near the liquidus (800.degree.-1000.degree. C.) is similar to that of water. This high fluidity, coupled with the proximity of the glass-transition (T.sub.g .about.320.degree. C.) and crystallization temperatures (T.sub.g .about.400.degree. ) in many HMFG compositions, translates into an oftentimes marked tendency for melts to devitrify or crystallize upon cooling. In addition, the presence of specific impurities such as hydroxyl chlorine species, and trace oxides may be instrumental in the nucleation and growth of crystallites.
These problems are often exacerbated by the melting techniques traditionally utilized to prepare HMFG which involve the use of oxide starting materials converted in situ to fluorides via heating with ammonium bifluoride or by the use of reactive atmospheres such as carbon tetrachloride/argon. While the conventional method represents a simple and straightforward approach to glass preparation, it has encountered difficulties in reproducing physical and optical properties from batch to batch. Moreover, such "conventionally" prepared samples often contain inclusions, crystallites, partial substitution of chlorine for fluorine or incorporation of particularly carbon dioxide, which degrades transmission near the 2350 cm.sup.-1 wavelength region.